1. Field of the Invention
This invention relates to bistable cholesteric displays and, in particular, although not necessarily solely, a method and apparatus for driving reflective bistable cholesteric displays.
2. Description of the Prior Art
Bistable cholesteric liquid crystal displays “BCD” are already well known and utilized in a number of applications. The bistable cholesteric display usually consists of two pieces of glass forming a thin liquid crystal cell. The liquid crystal material is usually composed of twisted nematic liquid crystal heavily doped with a chiral dopant to give the liquid crystal a strong twist sense or chirality. Such cholesteric liquid crystal displays exhibit two stable states at a zero driving voltage. The first of these is the reflective planar “P” state and, as the name suggests, is reflective in normal usage. The second state is the scattering/transparent focal conic (FC) state. In normal usage, the FC state is transparent.
When used in conjunction with a dark light absorber placed at the back of the display, the FC mate will appear black and the P state would appear to be bright when the display is viewed in reflection. The colour of the bright state can be adjusted by varying the chirality and pitch of the liquid crystal material. Such bistable liquid crystal displays are discussed in literature such as the monograph by S. Chandrasekhar entitled “Liquid Crystals” (Cambridge University Press, 1977).
A number of schemes have been developed to drive such bistable displays in a passive matrix manner. U.S. Pat. No. 4,571,585 by Stein et al. utilizes a wave form applied to the individual pixels which has been specially tailored in order to avoid cross talk problems. A number of voltage levels are needed for matrix driving of the display making the invention rather cumbersome. An alternative matrix driving scheme is disclosed in the document entitled “A High Information Content Reflective Cholesteric Display” (SID 95 Digest, 1995) by Pfeiffer et al. In this scheme, commercial LCD driver chips were used and the display was scanned with 20 ms pulses. An upper voltage of 41 V would give the P state and a lower voltage of 33 V would produce the FC state. The scanning of 20 ms per line was achieved. However, this scheme has the disadvantage of a high voltage requirement and the relative slowness in scanning.
A more complicated dynamic driving scheme is discussed in the document entitled “Cholesteric Reflective Display: Drive Scheme and Contrast” (Appl. Phys. Lett., 64, 1905, 1994) by Yang et al. In this scheme, a 1 ms addressing time was shown to be possible although was only provided at the expense of more complicated wave forms and driver electrons. Again, the voltages required were quite high at greater than 40 V.
A further drawback in the dynamic scheme of Yang et al. is the appearance of a dark band in the display. A yet further disadvantage is that the image does not appear instantaneously. Instead, there is a 300 ms delay due to the slow switching from the FC state to the P state. A yet further disadvantage is that the contrast ratio of the dynamic driving scheme is very te do the amplitude of the evolution voltage. The 1 ms addressing time shown to be possible at the expense of more complicated electronics is discussed in documents entitled “Dynamic Drive for Bistable Reflective Cholesteric Displays: A Rapid Addressing Scheme (SID 95 Digest, p. 347, 1995) and “High Performance Dynamic Drive Scheme for Bitable Reflective Cholesteric Displays” (SID 96 Digest, p. 359, 1996).
Faster switching to the P state has been demonstrated recently in a specially aligned BCD discussed in the document by M. H. Lu (Journal of Applied Physics, 81, 1063, 1997). Even with this faster switching available with a specially aligned BCD, it still takes approximately 10 ms for the switching.